Thermal processing of semiconducting wafers, such as silicon wafers, to produce semiconductor devices is well known in the art. The processing typically comprises maintaining a wafer at a different, known temperatures for predetermined times. During the processing, it is necessary to know the temperature of the wafer to a high accuracy in order to achieve repeatable results. Pyrometers, which measure temperature based on infrared emission and reflection from the wafer, are commonly used to monitor the temperature. Accurate pyrometric temperature measurement is dependent, however, on knowing the emissivity of the wafer, which varies with prior processing steps, temperature, and process steps performed while the measurement is being made.
Rapid Thermal Processing (RTP) is a state-of-the-art process used to manufacture semiconductor devices, wherein a wafer, or a region of a wafer, is cycled rapidly through a series of predetermined temperatures. Often rapid thermal processing is combined with chemical vapor deposition (RTCVD). Both processes rely on accurate, real-time determinations of the temperature of the region being processed. A review of RTP entitled Rapid Thermal Processing Science and Technology (ISBN 0-12-247690-5), Academic Press, California (1993), is herein incorporated by reference. Chapter 9 of the review indicates that processing efficiency is critically dependent on accurate temperature measurement of wafers during the processing.
In "Advances in Temperature Measurement and Control for RTP," by Peuse et al., 5th International Conference on Advanced Thermal Processing of Semiconductors--RTP 1997, which is incorporated herein by reference, the authors describe processing of semiconductor wafers using an RTP system known as the Centura system, produced by Applied Materials Inc., of Santa Clara, Calif. The system relies on placing the wafer in a highly reflective enclosure, so as to eliminate as nearly as possible the effects of variations in emissivity of the wafer, whereby the total enclosure closely approximates a black body. (The theory of pyrometric temperature determination by analysis of radiation from an object requires that an emissivity or an effective emissivity of the object be known; a black body has an emissivity of unity.) U.S. Pat. No. 5,490,728, to Schietinger et al., which is incorporated herein by reference, describes a non-contact pyrometric technique for measuring characteristics, including temperature, of a substrate. The technique estimates the emissivity of a wafer by comparing the amplitude of ripple flux in radiation reflected from the wafer with an incoming ripple flux amplitude due to an AC-powered heating lamp.
U.S. Pat. No. 5,255,286, to Moslehi et al., which is incorporated herein by reference, describes a method of optical pyrometry based on irradiating a semiconductor wafer with monochromatic coherent radiation at a wavelength of the order of 5 .mu.m. Intensities of reflected coherent radiation and emitted incoherent radiation from the wafer are used to determine a value of the emissivity and of the temperature of the wafer.
In "Optical Pyrometry in RTP/RTCVD Systems: A New Approach," by Glazman et al., 6th International Conference on Advanced Thermal Processing of Semiconductors--RTP'98, which is incorporated herein by reference, the authors describe a technique for measurement of the temperature of a semiconducting wafer using optical pyrometry. The technique uses an active multi-spectral system to dynamically measure the true emissivity of the wafer. The measured emissivity is utilized together with measurements of radiation emitted by the wafer in order to evaluate the temperature of the wafer.